There are different types of humidity sensors that detect the concentration of water vapor in a measurement target gas such as a resistance change type, a capacitance change type, a zirconia (ZrO2) solid electrolyte type, or the like. Particularly, in recent years, a zirconia thin film limit current type has attracted attention. This limit current type oxygen sensor has an advantage of high reliability and good linearity.
Furthermore, attempts have been made to apply the zirconia solid electrolyte type to, for example, an in-vehicle NOx sensor that measures the concentration of nitrogen oxide (NOx gas) in exhaust gas.
However, in the limit current type gas sensor of the zirconia thin film limit current type, the sensor characteristics need to be further improved and stabilized.